Interesting titbit: Modern Flashes trigger with roughly 5 volts signal across the contacts on the hotshoe. Old flashes can have any number of trigger voltages. Use of older flahsguns on modern cameras should therefore be dne with extreme caution. What prompted this topic? Well as an example we have a SunPak Auto 134 Flash Gun. Unfortunately the Hotsoe operates at 200V. and modern cameras usually put out a logic signal at 5 Volts.

So before you rush off to use an old flashgun on your brad new DSLR camera, stop, think and do some research. Buying a new flashgun is < buying a new camera.